Biogas Potential of OFMSW through an Indirect Method

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The usual tests for measuring the anaerobic biogas potential (ABP) of the Organic Fraction of Municipal Solid Waste (OFMSW) are reliable, but not practicable as a routine analysis. These types of analysis are expensive and time-consuming, requiring 21 (partial biogas production) to 100 days (total biogas production). Moreover, the operators of anaerobic digestion plants have often to choose which biomasses to use, depending on their digestibility and economical convenience. These matrices delivered daily to the plants, are heterogeneous especially with regard to the content of the biodegradable fraction, responsible for the potential production of biogas. In such a context, rapid methods toassess potential biogas productions are needed for helping operators in their choices. In order to contribute to solution tothese problems, in this work the Dynamic Respiration Index (DRI) was used to predict the anaerobic biogas potential. In this way, it is possible to know the ABP value in a short period of time (1–4 days). Two dynamic respirometric approaches were considered: the main difference between them is the temperature control (kept steady in one case, uncontrolled in the other one). In particular, DRI24 is determinated as the average integral over a period of 24 hours during the peak of maximum consumption in terms of oxygen. DRI24 is less affected by any oxygen consumption peak of limited duration. For these reasons, it wasrecommended to use its value to determinate the Potential Production of Biogas.A literature equation which determines the ABP from the value of DRI was modified in order to take advantage of the characteristics of the respirometric method with controlled temperature. About 130 Nm3/t of food waste was assessed for the studied area, thanks to this method. In this way, it was demonstrated that the plant operators can adopt aquick method to characterise the biogas potential of the OFMSW arriving daily at their plant, like a routine analysis.

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Biogas Potential of OFMSW through an Indirect Method

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Описание:

The usual tests for measuring the anaerobic biogas potential (ABP) of the Organic Fraction of Municipal Solid Waste (OFMSW) are reliable, but not practicable as a routine analysis. These types of analysis are expensive and time-consuming, requiring 21 (partial biogas production) to 100 days (total biogas production). Moreover, the operators of anaerobic digestion plants have often to choose which biomasses to use, depending on their digestibility and economical convenience. These matrices delivered daily to the plants, are heterogeneous especially with regard to the content of the biodegradable fraction, responsible for the potential production of biogas. In such a context, rapid methods toassess potential biogas productions are needed for helping operators in their choices. In order to contribute to solution tothese problems, in this work the Dynamic Respiration Index (DRI) was used to predict the anaerobic biogas potential. In this way, it is possible to know the ABP value in a short period of time (1–4 days). Two dynamic respirometric approaches were considered: the main difference between them is the temperature control (kept steady in one case, uncontrolled in the other one). In particular, DRI24 is determinated as the average integral over a period of 24 hours during the peak of maximum consumption in terms of oxygen. DRI24 is less affected by any oxygen consumption peak of limited duration. For these reasons, it wasrecommended to use its value to determinate the Potential Production of Biogas.A literature equation which determines the ABP from the value of DRI was modified in order to take advantage of the characteristics of the respirometric method with controlled temperature. About 130 Nm3/t of food waste was assessed for the studied area, thanks to this method. In this way, it was demonstrated that the plant operators can adopt aquick method to characterise the biogas potential of the OFMSW arriving daily at their plant, like a routine analysis.

represents a significant percentage (Eurostat,2013).

The European Council Directive on landfilling ofwaste(Directive1999/31/EEC)requiresmemberstatestoreducetheamountofbiodegradableMSW(BMSW)in landfills, over a 15year period, to 35% of theamount produced in 1995. Thanks to this legislation,new strategies for BMSW treatment for volumereduction, reutilization or energy recovery are andmust be developed. The European Directive2001/77/EC concerning the increase in the use ofrenewable energy sources in electricity production,was implemented through the D.Lgs.387/2003.Moreover, Italy has set itself the target in 2020 toproduce 17% of the total energy consumption fromrenewable sources and to replace 10% of energysourcesdesignedtotransport.

Theusualtestsformeasuringtheanaerobicbiogaspotential(ABP) of the Organic Fraction of Municipal Solid Waste(OFMSW) are reliable, but not practicable as a routineanalysis. These types of analysis are expensive and timeconsuming, requiring 21 (partial biogas production) to 100days (total biogas production). Moreover, the operators ofanaerobic digestion plants have often to choose whichbiomasses to use, depending on their digestibility andeconomical convenience. These matrices delivered daily tothe plants, are heterogeneous especially with regard to thecontent of the biodegradable fraction, responsible for thepotential production of biogas. In such a context, rapidmethods toassess potential biogas productions are neededforhelpingoperatorsintheirchoices.Inordertocontributeto solution tothese problems, in this work the DynamicRespiration Index (DRI) was used to predict the anaerobicbiogaspotential.Inthisway,itispossibletoknowtheABPvalue in a short period of time (14 days). Two dynamicrespirometric approaches were considered: the maindifference between them is the temperature control (keptsteady in one case, uncontrolled in the other one). Inparticular,DRI24isdeterminatedastheaverageintegralovera period of 24 hours during the peak of maximumconsumption in terms of oxygen. DRI24 is less affected byanyoxygenconsumptionpeakoflimitedduration.Forthesereasons,itwasrecommendedtouseitsvaluetodeterminatethe Potential Production of Biogas.A literature equationwhich determines the ABP from the value of DRI wasmodifiedinordertotakeadvantageofthecharacteristicsofthe respirometric method with controlled temperature.About130Nm3/toffoodwastewasassessedforthestudiedarea,thankstothismethod.Inthisway,itwasdemonstratedthat the plant operators can adopt aquick method tocharacterise the biogas potential of the OFMSW arrivingdailyattheirplant,likearoutineanalysis.

Oneofthesimpleandmostusedmethodsforbiomassenergyrecoveryisrepresentedbyanaerobicdigestionwith biogas generation(NallathambiGunaseelan et al.,1997;Anderottolaetal.,2000;Cecchietal.,2011).TheenhancementofOFMSWinanaerobicdigestionplantsallowsasignificantreductionofdisposalcosts,aswellas a strong reduction of environmental pollution byallowing parallel revenue from the sale of electricityandthermalenergyproduced.ThecompositionoftheOFMSWisextremelyheterogeneous,anddependsonmany factors such as seasonality, geographicallocationofthereferencebasin,theeatinghabitsofthepopulation.For example, in other studies(Hendriks etal., 2009), it is shown that in the summer, when thepercentages of MSW green waste increases(comingfrom mowing garden and public green areas), theyields in ABP of plants treating MSW are drasticallyreduced, and there is also a reduction of volatilesubstance.Thecauseofthedecreaseinproductivityisconnectedwiththeincreaseoftheligninandcellulosecontentinthetreatedsubstrate(Schievanoetal.,2010).Intheliterature,thenormalyieldoftheOFMSWintobiogasisapproximately130Nm3/t(Murphyetal.,2006;Piccinini et al., 2007). In Italy, the Italian Consortium

of Composters (CIC) hasproposed thata ABP range is

between90130Nm3/t(Giacettielal.,2011).

byconsideringthedynamicrespirometricindex(DRI),which provides information on the oxygenconsumption speed of the organic substance whichforms the biomass. The equation that permits todetermine the anaerobic biogas potential from thevalueofdynamicrespirometricindexis(Scaglia2010):

Anaerobic digestion plants operators have often to

choose which biomasses to use, depending on theirdigestibility and economical convenience. In thiscontext, rapid methods toassess potential biogasproductions are needed. Anaerobic biogasificationpotential (ABP) tests are reliable, but their responsetimes(from21daysforthepartialbiogasproductionto100 days for the total biogas production)areinadequate(Binner and Zach, 1999; Adani etal.,2001; Hansen et al.,2004). As the availability oforganicmatrixesonthemarketusetovarydaybyday,rapid evaluations of the ABP are needed. In theliterature,an indirect and faster method was assumedto evaluated the ABP or OFMSW through arespirometricapproach(Scagliaetal.,2010).Theresultcan be considered satisfactory as a compromisebetweenprecisionandrapidityoftheassessment.

34.4

0.109

0.003

(1)

inwhichtheABPandDRIareexpressedasNlkgDM1andmgO2kg DM1h1,respectively.Itwasdevelopedfrom RICICLA Group Di.Pro.Ve of the University ofMilan, through studies of 46 samples coming directlyfromMBTfullscaleplants.It is useful to do a regression on the variousmethodologies present in Italy for the determinationof DRI, to better understand what value to use in therelationship for the calculation of ABP. In Italy, thesector of respirometry applied to solid waste ischaracterizedbytheadoptionofdynamicapproaches,and above all there are two different methodologiesdevelopedinItaly.Ingeneral,thedynamicmethodtowhich the Italian regulations referis the Costech one,developedbyDi.Pro.Ve(UniversityofMilan).Parallelto this approach, in the University of Trento, anothersystem for measuring the dynamic respirability of asubstrate,theAIRnlrespirometer,hasbeendeveloped.

The present paperaimedto determine the biogas

potentialproductionfromOFMSWusingtheDynamicRespirometric Index values determined during thelaboratorytestscarriedoutwiththeAIRnlmethodinthe engineering laboratories of the University ofTrento(Scagliaetal.,2010;Dallago2002).Material and Methods

The AIRnlrespirometer is an adiabatic reactor having

one probe for temperature and one probe for oxygenmeasurements. The reactor is kept at the chosentemperaturebyathermostaticbath(30C).Thesystemof aeration and analysis is composed ofa doublepneumaticcircuit;forthisreasontheaerationissemicontinuous. In the first one, at low flow, the oxygenconcentration is measured in continuous, (alwayshigher than 18.5% (v/v)) (Andreottola et al.,2005);indeedthesecondone,athigherflow,guaranteesthereoxygenationbyacontinuousflowofairthroughthesample. An hourly dynamic respirometric index isassessed in mgO2 kgvs1 h1. Moisture in the sample iscorrectedto55%beforethetest,and1kgofsampleisusedforthetest.

In the year 2011 in Italy, 4.1mil.tons of OFMWS were

collected, while in the region of Trentino Alto Adige107,405 tons (Source: ISPRA 2012). In Figure 1, theOFMSW used for the development of the research ispresented. The final sample was obtained mixingabout three domestic sample of OFMSW collectedfromthetownofTrento.

FIGURE1:SAMPLESOFOFMSW

Regarding to the value of DRI better to use the value

ofDRI24;itisdeterminatedastheaverageintegralovera period of 24 hours during the peak of maximumconsumptionintermsofoxygen.DRI24islessaffectedby any peakoxygen consumption of limited durationin the highlighted respirometric path(Dallago 2002).Forthesereasons,itisrecommendedtousethevalueof IRD24 to determinate the Potential Production ofBiogas, because in this case the value that will beobtainedislesssensitivetoerror.

The ABP is an important parameter; from the

comparison of this data for different biomass, it ispossible to understand better which are the mostproductive in biogas generation. Furthermore, it isimportant to know not only the quantity of biogasproduction but also the production speed, and thebiodegradability of the substrate. The rate ofbiodegradation is important to know the times whenABPwillactuallybedeveloped.Thisismadepossible

monitored. The aim of the respirometric test is to

measure the respirometric activity by a quantitynamed dynamic respirometric index (DRI), whichmeasures the O2 consumption rate by the organicfractiondecomposingbacteria.ThisindexisexpressedasfunctionoftimeandrepresentsthevalueofspecificrespirometricactivitymgO2kgvs1h1(Dallago2002).

Given that the equation (1) uses the DRIDi.Pro.Ve, in

order to use the values obtained from the Trentomethod, the University of Trento has madeseveralexperimental tests with both methods to find arelationshipthatmakesitpossibletocomparethetwoDRI.Themaindifferencebetweenthetwomethodsisin control of the temperature during the test. In themethod developed by the group RECICLA,this is notexpected.Regarding data recorded with theCostechRespirometer, it was needed toreport oxygenconsumption to the reference temperature of 30C,inorder to compare the values with theones from theAirnlrespirometer. In literature, an expression hasbeenproposedvalidintherangebetween20and40C(Radaetal.,2012):

32 1000

(4)

ToestimatetheDRIvalue,itisnecessarytoknowsomephysicalcharacteristicsofthesamplesuchasmoisture,densityansVS.Tocalculatethemoistureofthesampleof OFMSW, the Moisture Analyzer HR83 has beenusedto determine the moisture content of almost anysubstance. The instrument works on thethermogravimetric principle: at the start of themeasurement the Moisture Analyzerdetermines theweightofthesamplewhichisthenquicklyheatedbythe integral halogen heating moduleand the moisturevaporizes. During the drying process, the instrumentcontinually measures the weight of thesample anddisplays the reduction in moisture. Once drying hasbeen completed, the moisture or solids contentof thesample is displayed as the final result.For thedetermination of VS, a furnace,model L9/11/SW wasused.Thedifferencebetweentheinitialtotalmassandthecombustionresidueisthecombustionloss.Duringthe process, the software includes records both thetemperature and the weight loss.Finally, to calculatethe density, the methodology ERRA (EuropeanRecovery & Recycling Association 1993) has beenapplied. It is one of the methodologies at EuropeanlevelforcharacterizeinquantitativelyandqualitativelyMSW.

(2)

From these studies(Rada et al.,2012) another

relationshipoffirstapproximationwithouttakingintoaccount the role of the temperature has been madeavailable, which allows correlating the two values ofDRI24.(3)

This equation shows that the method Di.Pro.Ve

underestimatestheoxygenconsumption,comparedtothatmeasuredwiththemethodAIRnl,moreover,theroleoftemperatureisanotherimportantparametertobe taken into consideration: the method AIRnl isbased on the concept of constant temperature duringthetest,unliketheDi.Pro.Vewherethetemperatureisnotadjusted.ExperimentalSetup:AIRnlTheadoptedreactorwasdevelopedinaPhDthesisatthe University of Trento, Italy. The project was basedonatypicalrespirometerforsludge,inwhichaprobemonitors the progressive decrease of present oxygenfromamaximumvaluenearsaturationtoaminimumset value, chosen for preventing limiting conditions.Whenitreachesitsinferiorvalue,asmallcompressorstarts automatically to recirculate the air andestablishes the initial oxygen value in order for theprocess to be repeated. In this way, the oxygenvariations curve permits obtaining a detailedrespirometerdiagram which is not limited in time.During the respirometric experiments, thetemperature and the volumetric percent of oxygen inthe air present in the system are continuously

whereTreferstoinstantaneoustemperatureatwhichthe measure is done (with the temperature probeplaced into the substrate)andisreference constant,equalto1.08.

0.7086

ResultsThe physical characteristics of the treated samplearepresentedinTable1.Thesevaluesweredeterminedbymeasurement in the laboratories of the University ofTrento.TABLE1:PHYSICALCHARACTERISTICSOFTHESAMPLES

in the expected value, may be given by the period in

which the sample of OFMSW was collected. Sampleswere collected in winter, when the content of theputrescible fraction in OFMSW is very small, becausethe householders eat less vegetables than that at thesummer or spring time. For this reason, other valueswere selected for use from other studies made by theUniversityofTrentoonothersamplesofOFMSW.

Inthepresentpaper,onesampleoforganicfractionofmunicipal solid waste (OFMSW) collected in Trentowas analysed. Three respirometric tests wereperformedforbetteremphasizingtheobtainedresults,due to the heterogeneity of the materials. The resultsfortheDRIandDRI24variationareshowninFigure2.Test 1

Test 2

Test 3

1400

800

Through the relationship (2) and (3) seen in the

previous paragraph, it was determined that, fromDRI24 AIRnl values, an average value of DRI24Costechisequalto3,805mgO2kgvs1h1.

600

TABLE3:RESULTSABOUTAIRNLANDCOSTECHMETHOD

-1h-1]

1200

RI [mgO2 kgSV

1000

Sam

400

200

123

00

10

20

30

40

50

60

70

80

Time [h]Test 1

Test 2

-1h-1]

1200

RI24 [mgO2 kgSV

Aver.Temp.[C]31.130.430.4

DRI24(ref.T:Costech)[mgO2kgVS1h1]5,2565,4745,383

DRI24Cos AveragtecheTemp.[mgO2kg[C]VS1h1]3,72439.53,87839.03,81439.2

Thisvalueisconsistentwithothercharacteristicvaluesof this matrix obtained by other studies (Scaglia etal.,2008). The values of the potential production ofbiogas(ABP)ofsuchsamplesofOFMSWanalyzedarereported in the following Table 4, determined by theequation(1).

ThisOFMSWshowsahighdegreeofputrescibilityandmoisture(greaterthan60%)thatmakesthemsuitableto anaerobic digestion. The application of anaerobicdigestion to waste treatment allows both to produce,throughtheaerobictreatmentofthedigestedsludge,aresidue stabilized employable as organic fertilizer inagriculture, and to achieve a considerable energyrecovery, through the use of biogas produced. Theappearanceoftheenergyrecoveryisundoubtedlythemost interesting. The biogas produced consistsmostlyofmethane(5075%,butnormallythevalueofCH4is60%) (Sosnowski et al., 2003),it has a high calorificvalue (approximately of 5.5 kWh/Nm3) (source:ENAMA) and therefore it can be convenientlyconverted to almost all forms of useful energy: heat,electricity and cogeneration.From the statistical dataobtainedfromtheportaloftheprovinceofTrento,itisseen that the amount of OFMSW from separatecollection of municipal waste is equal to 47,098 tonsfortheyear2011(STATweb,2013).Fromstudiesinthe

DRImed DRI24med[mgO2kgvs1h1]

1,164.99 1,103.34

The OFMSW is composed ofdifferent types of

biodegradablewastespresentinthehouseholdwaste.Normally, the value of DRI for food waste is around20004000mgO2kgvs1h1(Scaglia et al.,2008). Thedifferenceswhichappearforthetestperformedonthesametypeofsamplearecausedbytheheterogeneouscharacter of the OFMSW. A reason for this difference

literature(Piccinini et al.,2008), it appears that by the

treatment of the biogas cogeneration the followingquantities of electricity and thermal energy from 1m3of biogas can be obtained: from 1.8 to 2 kWh ofelectricityenergy,and23kWhofheatthermalenergy.If all the OFMSW produced inProvince of Trentoin2011 wereaddressed to the formation of biogas, itwouldbepossibletoobtain12,236MWheland16,083MWht,aspresentedintheTable5andTable6.

waste,

Management

OFMSW

Prod.

ABPave

BiogasProd.

[Tons]

[Nm3/t]

[Nm3]

47,098

136.7

6,440,180.2

OFMSW

International

Waste

Landfill

Symposium.

Santa

Binner, E., Zach, A.Biological reactivity of residual wastes

and dependence on the duration of pretreatment.WasteManagementandResearch17(1999):543554.Cecchi,F.,Bolzonella,D.,Pavan,P.,Mac,S.,MataAlvarez,J.Anaerobic Digestion of the Organic Fraction ofMunicipalSolidWasteforMethaneProduction:Researchand Industrial Application, Reference Module in EarthSystems

and

Environmental

Sciences,

from

ComprehensiveBiotechnology,6(2011):463472.Dallago, L. Development and application of respirometric

TABLE6:VALUESOFELECTRICITYANDTHERMALENERGYPRODUCTION

Samp.

and

of

MargheritadiPula,Cagliari,Sardinia,Italy,2005.

TABLE5:VALUESOFBIOGASPRODUCTION

Samp.

Proceedings

BiogasProd.

ElectricityProd.

ThermalProd.

techniques to evaluate the stability of solid matrices in

[Nm3]

[MWhel.]

[MWht.]

the composting phase, PhD diss, University of Trento,

6,440,180.2

12,236.3

16,083.9

2003.Direttiva 1999/31/CE del Consiglio, del 26 aprile 1999,

Conclusions

relativaallediscarichedirifiutiGazzettaufficialen.L182

During this study, it emerged that the relation

between the values of ABP and the DRI is a usefulmethod to determine in a short time the value of thepotentialproductionofbiogasinthecaseofOFMSW.Moreover, as the composition of the organic matrixvaries in both geographically and seasonally, rapidmethods to determine the potential in biogas areneeded. Through this report, the waiting time toobtainthisinformationisshorter.Itpassesfrom2160days to just 14 days, time needed to conduct arespirometrictest.Inthisway,itmaybepossiblefortheplant operators to use this method to value theOFMSWmoreproductiveintobiogasproductionthatarrivedailyattheplant,likearoutineanalysis.

digestion of sewage sludge and organic fraction of

municipal solid wastes. Advance Environmental

ISPRA Istituto Superiore per la Produzione e la Ricerca

STAT.web (statistic system of Autonuoun Province of

2013.http://www.isprambiente.gov.it/it/archivio/eventi/a

Trento). Accessed March 2013. http://www.stat.web.

nno2012/presentazionerapportorifiuti2012.

provincia.tn.it.

StefanoAntognonigraduatedinEnvironmentalEngineeringat the University of Trento in February 2013, with a thesisentitled Biological stability and potential energy of solidbiomass waste. Since April 2013, he has collaborated withthe Department of Civil and Environmental Engineering ofUniversityofTrento.

Elena Cristina Radagraduated in 2002 in Energetic

Engineering (environmental and economical career) in theDepartment of Environmental Engineering of University ofBucharest, Romania. She is coauthor of more than 200technicalscientific works published at national andinternational level concerning the following research areas:MSW selective collection and treatment, biomass,energyrecovery,contaminated sites remediation techniques; andenvironmentalimpact(andhealth).